Field
The present invention relates to microelectromechanical devices and especially to a microelectromechanical sensor device, as defined in the preamble of the independent claim.
Description of Related Art
Micro-Electro-Mechanical Systems, or MEMS can be defined as miniaturized mechanical and electro-mechanical systems where at least some elements have a mechanical functionality. MEMS structures can be applied to quickly and accurately detect very small changes in physical properties. As an example, a microelectromechanical gyroscope can be applied to quickly and accurately detect very small angular displacements.
Motion can be considered to have six degrees of freedom: translations in three orthogonal directions and rotations around three orthogonal axes. The latter three may be measured by an angular rate sensor, also known as a gyroscope. MEMS gyroscopes use the Coriolis Effect to measure the angular rate. When a mass is moving in one direction and rotational angular velocity is applied, the mass experiences a force in orthogonal direction as a result of the Coriolis force. The resulting physical displacement caused by the Coriolis force may then be read from, for example, a capacitively, piezoelectrically or piezoresistively sensing structure.
In MEMS gyros the primary motion cannot be continuous rotation as in conventional ones due to lack of adequate bearings. Instead, mechanical oscillation may be used as the primary motion. When an oscillating gyroscope is subjected to an angular motion orthogonal to the direction of the primary motion, an undulating Coriolis force results. This creates a secondary oscillation orthogonal to the primary motion and to the axis of the angular motion, and at the frequency of the primary oscillation. The amplitude of this coupled oscillation can be used as the measure of the angular rate.
Gyroscopes are very complex inertial MEMS sensors. The basic challenge in gyroscope designs is that the Coriolis force is very small and therefore the generated signals tend to be minuscule compared to other electrical signals present in the gyroscope. Spurious resonances and susceptibility to vibration plague many MEMS gyro designs.
One challenge in gyroscope design is quadrature error motion. In an ideal gyroscope structure, the primary oscillation and the secondary oscillation are exactly orthogonal. However, in practical devices imperfections occur, causing direct coupling of the primary mode displacement of the seismic mass to the secondary mode of the gyroscope. This direct coupling is called the quadrature error. The phase difference between the angular motion signal and the quadrature signal is 90 degrees, which means that basically the quadrature error could be eliminated with phase sensitive demodulation. However, the quadrature signal can be very large in comparison with the angular motion signal, and may therefore cause unreasonable dynamic range requirements for the readout electronics or phase accuracy of the phase demodulation.
One known method to deal with this error source is electrostatic quadrature cancellation that removes the error signal at the sensor structure, before the quadrature signal is generated. For this, an electrostatic force, exactly in-phase with the primary oscillation and parallel to the secondary oscillation may be applied to the seismic mass.
Electrostatic quadrature suppression is a very effective and therefore widely used technique. It can also be easily combined for even higher performance with electronic quadrature cancellation and other processing methods in the integrated circuit side. However, advanced gyroscope structures may be complicated and the microfabrication tolerances may be poor compared to their dimensions, so voltages necessary to compensate the quadrature component in the drive motion may be very high. This tends to complicate electronics design and increases power consumption of the gyroscope device.